There was link to the paper [1] on the list, where authors investigate
possibility of fault-tolerant btree using shadowing techniques, which
allows writable snapshots. Probably it will be of interest to you.
It's on my list for testing, but i haven't any benchmarks done with
described things yet.
[1]http://www.cs.tau.ac.il/~ohadrode/papers/btree_TOS.pdf
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DU